1. Field of the Invention
The present invention relates generally to the field of gas pumps, and, more particularly, to a thermal transpiration pump, or compressor. Although the present invention may subject to a wide range of applications, it may be especially suited for use in portable and in situ instruments. Throughout the text, pump, vacuum pump, and compressor will often be used interchangeably in describing embodiments of the invention.
2. Description of the Related Art
Pumps are required for the evacuation of a gas space or to draw in a gas sample from the local ambient environment in numerous systems. Many of these systems, such as, gas sensors, chemical reactors, and electron optical devices are important research instruments that may need rough (760 Torr-1 mTorr), high (1 mTorr-10xe2x88x927 Torr) and possibly ultra-high (10xe2x88x927 Torr-10xe2x88x9211 Torr) vacuum levels in order to function properly. A great number of these instruments are in the process of being miniaturized into microelectromechanical systems (MEMS) in order to take advantage of the smaller resultant system mass, volume, and power consumption usually obtained through miniaturization. The fabrication of MEMS devices are achieved utilizing processing techniques and equipment developed for the integrated circuit industry. The benefits miniaturization enables has led to commercial as well as governmental interest in the development of portable, integrated analytical instruments. Mass spectrometers, optical spectrometers, gas chromatographs, electron beam optics and on-demand gas generators are some of the devices in various stages of MEMS development.
The currently available commercial rough pumps may not meet the requirements imposed by the miniaturized analytical instruments proposed for portable use. Commercial pumps are typically oversized for the small flows needed and pose serious technical challenges towards their miniaturization. In these small instrument systems the pump tends to have the greatest mass, volume, power consumption and cost penalties. Obviously, miniaturized and possibly integrated pumps would greatly increase the attractiveness of these types of instruments. The few existing miniaturized pumps, for example, mesoscale pumps and micropumps, suffer from negative scaling issues, poor performance, constrained operation or detrimental system impacts. Pump selection for portable and in situ instruments heavily depends on the system""s requirements for contamination, noise, vibration, reliability and on the budget allocations for power, size and mass.
More recently several thermal transpiration pump configurations have been suggested. Thermal transpiration describes the regime where gas flows can be induced in a system by maintaining temperature differences across orifices, porous membranes, or capillary tubes under rarefied conditions. The driving reason for these attempts has been the attraction of a pump or compressor with no moving components and no fluids. The major disadvantages are that such pumps have low volume flow rates and tend to be energy inefficient. A further drawback is a low pressure limit for pumping significantly lower than 1 mTorr. The limit appears because it is generally assumed that certain critical pump components must be at least a factor of ten larger than the molecular mean free path of the working gas.
A need therefore exists for a pump for miniaturized system usage that can operate under free molecular conditions over a wide and useful pressure range.